specialty, exclusive, mainly, solely
專 centers on concentrated focus on one thing: a specialty, something exclusive, or doing something mainly or solely.
專 is the traditional form of 専. It is thought to combine 寸 (hand) with a phonetic element that also suggests turning or concentrating, giving the idea of focusing on one thing.
The bottom 寸 looks like a hand holding something, and the dense upper part suggests a single object being turned over and over in the hand, focusing on it exclusively.
For セン, imagine a specialist sending a single, focused message: send -> セン, and the specialist works solely on that one task.